1. Field of the Invention
This invention relates to a method of and apparatus for automatically controlling travel of two film strips of a thermoplastic synthetic resin, such as vinyl or polyethylene, as a bag or package material, which are intermittently moved by an intermittent conveyor means, and more particularly to such method and apparatus that can be applied to a method and machine for manufacturing bags from such material and/or for packaging articles with the material.
2. Description of the Prior Art
It is well known that in machines for manufacturing bags or the like from two plastics film strips, such strips are processed by processing devices such as a heat-sealing device and a cutting device at each inoperative interval of the intermittent conveyor means of the nipper type which comprises a pair of rollers. As the film strips are moved in transversely spaced parallel relationship by such conveyor means, it occurs necessarily that either one of the strips goes ahead of the other, thereby producing relative positional gaps or discrepancies between the paterns, letters, trademarks and the like which are printed on the strips at regular intervals. Such gaps or discrepancies tend to become larger and larger as the machine is continuously operated. Accordingly, if and when the strips are processed by the processing devices without correcting such gaps or discrepancies, a considerable amount of incomplete or defective materials are produced.
In a conventional method and apparatus for controlling travel of the two transversely spaced parallel film strips, travel of the strips is controlled in such a way that either one of the strips that goes ahead of the other is retarded by an electromagnetic retarding device for the strip which is arranged to be actuated in response to detection by a sensor of the register marks which are indicated on the strip. With such a way of control, however, the relative positional gaps or discrepancies between the register marks on the travelling strips cannot be rapidly and effectively corrected, since the amount of retardation of a travelling film strip obtained in such a way is very small relative to the distance for which the strip is moved at a time by the intermittent conveyor. In fact, if the machine is of the type which can transport the strips for 20 cm, the amount of retardation of each of the strips obtained by such retarding devices is to the order of 1 mm. Accordingly, if there is a relative positional gap of 5 mm to 10 mm long between the register marks on the strips, the retarding force must be frequently applied to the preceding strip by the retarding device in order that the relative positional gaps between the register marks on the strips can be corrected. Thus, in accordance with the conventional method and apparatus, waste of material cannot be avoided.